The glass art has devised numerous tests to measure the durability of a glass, i.e., the resistance of the glass to attack by such chemical agents as acids and bases. The majority of these tests utilizes two general thrusts, viz., either a quantitative determination of the change in weight of a particular sample before and after exposure to the chemical agent or an analysis of the leachant or attacking agent to learn which elements and how much thereof have been removed from the sample.
However, consumer studies have indicated that most frequently qualitative changes in appearance are viewed by the customer as indicative of the chemical durability of a product, rather than any weight loss resulting from corrosion. Upon reflection, such a finding would be self-evident since the consumer is normally only interested in the aesthetic appearance of a particular product and is not concerned with minuscule losses in weight thereof unless, of course, such losses give rise to the development of porosity or other flaws in the surface of the product. One such qualitative change in appearance, viz., iridescence, is a common phenomenon appearing in glassware subjected to strong detergent solutions (bases) of the type employed in home dishwashers or commercial dishwashers in food service facilities, e.g., restaurants, hospitals, schools, etc.
The mechanism underlying the development of iridescence is believed to involve the chemical leaching of the glass surface. Hence, the attacking solution selectively dissolves out certain components of the glass, leaving behind a porous network of material (in the common silicate glasses of commerce the network material will be silica-rich), the thickness and refractive index of which lead to the production of the various blue, purple, and yellow interference colors distinctive of iridescence. It has been learned that many factors contribute to the initiation and extent of chemical attack suffered by a particular glass. When dealing with detergent solutions, significant elements to be considered affecting the rate of leaching include the type and concentration of anions in the solution, the temperature, and the pH of the solution.
Among the most prevalent anions encountered in commercially-marketed detergents are phosphates, polyphosphates, various soluble silicates, carbonate, and bicarbonate. Consequently, because it is not known which type of detergent will be utilized by the consumer, the glassware to be used in food service applications must be capable of resisting attack by all of those anions.